Fixed #24503 -- Added docs on LANGUAGE_CODE fallback change in 1.8.

* If the locale middleware isn't in use, it decides which translation is served

  to all users.

* If the locale middleware is active, it provides the fallback translation when

  no translation exist for a given literal to the user's preferred language.

* If the locale middleware is active, it provides a fallback language in case the

  user's preferred language can't be determined or is not supported by the Web

  site. It also provides the fallback translation when a translation for a

  given literal doesn't exist for the user's preferred language.

.. versionchanged:: 1.8

    The fallback for translation literals was added.

See :ref:`how-django-discovers-language-preference` for more details.